Currently military numbers are limited by manpower. And military units (like let's tanks) are to some degree limited by money. You don't want to pour too much money on single tank, because you then put all your eggs on few baskets. And you also don't want to put significant part of your troops in badly armored tanks with food firepower, because then you will run out of tankers in no time. Currently the "good balance" is 10 000 M1A2 Abrams that cost 6,2 million dollars a piece. Those would need 40 000 tankers, which is tolerable amount of crew. Training them is costing and going to cost lots of money. Combined price of the thing is 62 billion dollars or tax payer money, distributed to several decades. Increasing the price of tank to 10 million would be stupid, as it's still vulnerable to certain dangers, but you now lose 4 million more in single blast.
Now let's imagine full autonomous tank fleet of M1A9 Abrams somewhere around year 2050. You are no longer limited by tankers. You are only limited by price. But as you lack crew, you need less armor. Lets say we have unit price of 4 million dollars. Now 10 000 units is only 40 billion and zero tankers.
Then Russia makes autonomous tank "T17 Kursk" and produces 15 000 of them to sell to China. (Or something else, I'm not fiction writer.) They are bit crap, but as trained tankmen are not required, they can easily produce higher numbers to compete with U.S.
But U.S. is not happy with second place. So U.S. gets another 10 000 Abrams.
But China is not giving up, they order another 15 000 Kursk.
And on and on. Until tax percentages of both China and U.S. reach 70% and all of it is poured to "defense". China, India, Brazil and Russia are all likely to beat U.S. in economic growth rate in the future, because it's easier if you start from the bottom. And they are all geopolitically ambitious. We are headed towards multipolar world and that means arms races again.
TL;DR: Autonomous killing robots remove limitations of arms race. This is "destabilizing" and draining.